An example of a radiation detector for detecting radiation such as X-ray includes a semiconductor detector using a semiconductor. A semiconductor detector often needs to be cooled by liquid nitrogen, while a silicon drift detector (SDD) may reduce noise without being cooled to the liquid nitrogen temperature. An interface between Si and SiO2 is present around the surface of the semiconductor detector using silicon (Si), and electric charges irrelevant to electric charges derived from the entrance of radiation occurs at the interface. The occurrence of electric charges causes surface current, which may be a source of noise. Japanese Patent Application Laid-Open Publication No. 2014-2155 discloses a technique of collecting electric charges generated around the surface of an SDD. As an example, Japanese Patent Application Laid-Open Publication No. 2014-2155 discloses an SDD having a river structure in which a part of a ring-shaped electrode formed on a surface is made discontinuous in order to collect electric charges.